SiC single crystals are thermally and chemically very stable, superior in mechanical strengths, and resistant to radiation, and also have superior physical properties, such as high breakdown voltage and high thermal conductivity, compared to Si single crystals. They are therefore able to exhibit high output, high frequency, voltage resistance and environmental resistance that cannot be realized with existing semiconductor materials, such as Si single crystals and GaAs single crystals, and are considered ever more promising as next-generation semiconductor materials for a wide range of applications including power device materials that allow high power control and energy saving to be achieved, device materials for high-speed large volume information communication, high-temperature device materials for vehicles, radiation-resistant device materials and the like.
Various strategies have been used in the attempt to develop SiC single crystals that can be used for such purposes. For example, it has been attempted to obtain SiC single crystals with low volume resistivity (hereunder referred to as “resistivity”) in order to reduce electric power loss when such SiC single crystals are to be applied in electronic devices, such as power devices. For example, a SiC single crystal is doped with an n-type impurity to reduce the resistivity of the SiC single crystal, and nitrogen may be used as the n-type impurity (PTL 1).
Typical growth processes for growing SiC single crystals that are known in the prior art include gas phase processes, the Acheson process, and solution processes. Among gas phase processes, for example, sublimation processes have a drawback in that grown single crystals have been prone to hollow penetrating defects known as “micropipe defects”, lattice defects, such as stacking faults, and generation of polymorphic crystals. However, most SiC bulk single crystals are conventionally produced by sublimation processes because of the high crystal growth rate, with attempts being made to reduce defects in the grown crystals. In the Acheson process, heating is carried out in an electric furnace using silica stone and coke as starting materials, and therefore it has not been possible to obtain single crystals with high crystallinity due to impurities in the starting materials.
Solution processes are processes in which molten Si or an alloy melted in molten Si is situated in a graphite crucible and C is dissolved into the molten liquid, and a SiC crystal layer is deposited and grown on a seed crystal substrate set in the low temperature zone. Solution processes can be expected to reduce defects compared to sublimation processes, since crystal growth is carried out in a state of near thermal equilibrium, compared to gas phase processes. In recent years, therefore, several methods for producing SiC single crystals by solution processes have been proposed, and methods for obtaining SiC single crystals with few crystal defects have also been proposed (PTLs 2 and 3).